gas mileage", so in other words, "incredibly boring".
So obviously the correct choice is a 2000 Mercedes Benz e320. They go for around $6k, rated at 30mpg, plenty of room and power, comfortable, cheap 16" wheels, excellent build quality, and easy to work on. Not to mention side front and rear curtain air-bags are standard (find that in a Civic for under $6k!), and insurance isn't bad either.
When I was looking for my commuter car, my wife saw my 5 and 7 series searches and told me to get something "reliable and economical", so I picked up an e320. Good grief, the two front tires I just replaced were $180 out the door (mounted balanced, etc), that's almost the cost of a single 540i tire.
The car is "fun enough", but she is no M-sport, but I don't deny 30mpg is nice, and my ride is smoother and quieter than a civic, and the engine is easier to work on than any FWD (and the V6 is located for easy access).
That said, I'm still on the lookout for an 850i 6-speed for my weekend warrior/project!